1. Field of the Invention
The present invention relates to an image forming apparatus, a method of controlling the apparatus and a program for implementing the method, and more particularly to a technique of controlling a beam which is pulse width modulated based on an image signal and irradiated onto a photosensitive member to form an electrostatic latent image thereon.
2. Description of the Related Art
In a conventional image forming apparatus, such as an electrophotographic printer or a copying machine, generally, an electrostatic latent image is formed on a photosensitive member by irradiating a beam (laser beam), which is modulated based on an image signal, onto the photosensitive member, and this electrostatic latent image is developed by toner.
A pulse width modulation (PWM) method is known as a laser driving method, in which the duration of irradiation of a laser beam per unit pixel is modulated according to the image signal while the luminous intensity of the laser beam (the amount of laser beam) is maintained constant.
In the PWM method, the luminous intensity of the laser beam has to be maintained constant. However, a laser emitting device that emits the laser beam has a temperature characteristic such that the higher the temperature of the laser emitting device, the greater the amount of driving current required to maintain the luminous intensity of the laser beam constant. Moreover, the laser emitting device itself generates heat, and therefore the luminous intensity cannot be maintained constant merely by applying a constant amount of current to the laser emitting device. Thus, even if an image is formed by a PWM signal corresponding to an image signal having a uniform density, the resulting image has a non-uniform density.
To eliminate such non-uniform density of an image, means for adjusting the luminous intensity of the laser beam to a constant value has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H05-145154 and Japanese Laid-Open Patent Publication (Kokai) No. 2000-339736).
An image forming apparatus having the means for adjusting the luminous intensity of the laser beam to a constant value includes, for example, a laser chip 43 comprised of a laser emitting device 43A, and a photodiode (hereinafter referred to as “PD sensor”) 43B, as shown in FIG. 10. Current from a bias current source 41 and a pulse current source 42 are superimposed, the resulting sum current is supplied to the laser emitting device 43A, and a laser beam from the laser emitting device 43A is photoelectrically converted into a detection signal by the PD sensor 43B. The detection signal (current) is fed back to the bias current source 41, whereby the amount of bias current (reference current amount), that is, the luminous intensity of the laser beam is controlled to a constant value. This control of the bias current amount is performed whenever main scanning is carried out but before image formation is carried out.
Specifically, a sequence controller 47, upon input of a synchronization signal for synchronization of the main scanning, outputs a full light signal to an OR gate 40 to turn a switch 49 on. Upon turn-on of the switch 49, the sum of current from the bias current source 41 and current from the pulse current source 42 flows to the laser chip 43, and at this time an output signal (current) from the PD sensor 43B is input to a current-to-voltage converter 44 where the output signal is converted into a voltage signal. This voltage signal is amplified by an amplifier 45 and the amplified voltage signal is input to an APC (Auto Power Control) circuit 46. The APC circuit 46 compares the input voltage signal with a reference voltage and generates a control signal for controlling the bias current amount to a constant value, depending upon the comparison result, and supplies the control signal to the bias current source 41. The above type of circuit is an APC circuit type, which is generally used as a type of circuit for driving a laser beam.
After thus adjusting the bias current to a constant value, the sequence controller 47 turns off the full light signal. The switch 49 is then switched on and off by a PWM signal which is modulated based on the image signal by a modulator 48, so that the pulse current based on the image signal is superimposed upon the bias current, and the resulting sum current is fed to the laser emitting device 43A, whereby an electrostatic latent image is formed on the photosensitive member.
However, even if the photosensitive member is irradiated by a laser beam controlled to a constant luminous intensity, potential irregularities of 5V or more (surface potential irregularities and electrostatic latent image potential irregularities) can occur in both the main- and sub-scanning directions due to irregularities in the surface characteristics of the photosensitive member. Such potential irregularities result in non-uniformity of the density of the image formed.
One of the causes of such potential irregularities is irregularities of the thickness of the thin film of the photosensitive member. It is very difficult to manufacture a photosensitive member with a uniform thin film thickness. A non-uniform thin film of the photosensitive member typically results in unevenness in pattern between main scanning lines. Such a photosensitive member, when used, causes density irregularities of the image formed in the longitudinal direction on the image plane, degrading the image.
Moreover, improper assembly of component parts of the image forming apparatus around the photosensitive member, such as variations in the gap between the charging device and the photosensitive member or in the gap between the photosensitive member and the developing device or inclination of the charging device or the developing device relative to the photosensitive member, causes potential irregularities in the main scanning direction. Such potential irregularities also results in unevenness in pattern between main scanning lines.